1. Field of the Invention
The present invention generally relates to a taste sensing method using artificial lipid membranes.
The present invention also relates to a technique for detecting and measuring a difference in taste of beverages and comestibles, which is formerly difficult to measure by an artificial sensor, by utilizing a sensor that can replace the sense of taste as one of the five senses of human.
Furthermore, the present invention relates to a technique for detecting a difference in taste of food, e.g., beverage and alcohol goods to drink. Hence, the present invention relates to a technique which enables quality control of beverages and alcohols at the factories by an automated machine system without using a human labor.
2. Description of the Related Art
Basic taste elements are said to consist of saltiness, sweetness, bitterness, sourness, and "Umami". These elements are exibited in different degrees, respectively. Let us suppose that a difference between these tastes that can be evaluated by human senses or a difference between different degrees of the same taste, e.g., saltiness that can be evaluated by human senses, can be grasped as a physically measurable amount, and that a measurable taste or a measurable difference in taste (comparative or relative taste) is referred to as taste in this specification.
The present applicants have previously filed a patent application of an invention entitled "Taste Sensing System Using Artificial Lipid Membranes" (U.S. patent application Ser. No. 07/555,163). The specification and its accompanying drawings show that a lipid molecular membrane, having a matrix structure in which a lipid material comprising hydrophobic molecules and hydrophilic molecules is fixed in a polymer matrix and the hydrophilic molecules of the lipid are arranged in the matrix surface, can serve as a taste sensor that can replace the human sense of taste.
The outline of such a taste sensor will be briefly described. Namely, assume that a partial description of the above application is incorporated hereinafter.
FIG. 9 shows a schematic view of formation of such a lipid molecular membrane by an expression method used in a designing method of a chemical substance. Each lipid molecule shown in FIG. 9 includes a hydrophilic group a, i.e., a hydrophilic portion a represented by a spherical portion which is indicated by a circle, and a chain structure b (e.g., an alkyl group) of a hydrocarbon in which an atomic array extends. In FIG. 9, two chains extend to represent one molecule, thereby constituting molecule group as a whole. This chainportion of the hydrocarbon is a hydrophobic portion b. These lipid molecules 31 are received in a surface structure of a matrix 33 of a membrane material 32, i.e., in the surface of a planar wide micro structure and inside the matrix 33 so that they are dissolved therein (e.g., 31' in FIG. 9). The molecules 31 are accommodated such that the hydrophilic portions are arranged on the surface.
FIGS. 10A and 10B show a multi-channel taste sensor using such a lipid molecular membrane. FIGS. 10A and 10B show three sensing parts of multi-channel electrodes.
In FIGS. 10A and 10B, holes having a diameter of 1.5 mm are formed in a base material, and silver rods are inserted in the holes to provide electrodes. The lipid molecular membrane is applied on the base member to contact the electrodes through a buffer layer.
FIG. 11 shows a taste measuring system using the multi-channel taste sensor described above.
An aqueous solution of taste substances was prepared and put as a solution 11 to be measured in a vessel 12 such as a beaker. A taste sensor array 13 manufactured by arranging lipid membranes and electrodes on an acrylic plate (base member) as described above was put in each solution 11 to be measured. Before the sensor array 13 was used, an electrode potential was stabilized by soaking in an aqueous solution of potassium chloride having a concentration of 1 m mol/l. In FIG. 11, black dots 14-1, . . . , 14-8 represent the lipid membranes.
A reference electrode 15 was prepared as an electrode for generating a reference potential of measurement and put in the solution to be measured. The taste sensor array 13 and the reference electrode 15 were separated from each other by a predetermined distance. The surface of the electrode 15 was covered with a material prepared by fixing potassium chloride having a concentration of 100 m mol/l in agar-agar as a buffer layer 16. Therefore, the electrode system is constituted by silver 2 .vertline. silver chloride 4 .vertline. lipid membrane 3 (14) .vertline. solution to be measured 11 .vertline. buffer layer (potassium chloride 100 m mol/l) 16 .vertline. silver chloride 4 .vertline. silver 2.
Electrical signals from the lipid membranes are supplied as 8-channel signals to buffer amplifiers 19-1, . . . , 19-8 via lead wires 17-1, . . . , 17-8, respectively. Outputs from the buffer amplifiers 19 are selected by an analog switch (8 channel) 20 and loaded to an A/D converter 21. An electrical signal from the reference electrode 15 is also supplied as a reference potential to the A/D converter 21 via a lead wire 18. A difference between the reference potential and a potential from the membrane is converted into a digital signal so as to process in a micro computer 22 and display on an X-Y recorder 23.
In this example, an 8-channel taste sensor is used. The channels comprise a lipid molecular membrane shown in Table 1 which have different response characteristics with respect to a taste in order to obtain large number of taste information, thus reproducing the human sense of taste.
TABLE 1 ______________________________________ No. Name (Abbreviation) ______________________________________ 1. dioctylphosphate (2C.sub.8 POOH) 2. cholesterol 3. trioctylmethyl ammonium chloride (TOMA) 4. oleic acid 5. n-octadecylchloride 6. diphenyl phosphate 7. decylalcohol 8. dioctadecyldimethylammonium bromide (DOAB) 9. lecithin 10. trimethyl stearyl ammoniumchloride (TMSA) 11. oleylamine ______________________________________
It is assumed that an electrode of the taste sensor has a potential profile as shown in FIG. 12. In FIG. 12, an intramembrane potential gradient is expressed as 0 (positive and negative charges are uniformly distributed in the membrane).
When a taste is to be detected and measured by using the lipid molecular membrane (also called simply as a lipid membrane) described above, several new countermeasures are demanded.
For example, when taste quality control is to be performed for discriminating foods having only slightly different tastes or manufacturing foods whose only slightly different tastes must be discriminated, a difference in taste sensor outputs becomes small accordingly. For this reason, a measured value obtained by the sensor must have good reproducibility, and a measured value must not vary.
However, with the conventional taste measurement using the lipid membrane described above, such new demands cannot be satisfied.